THE SEA FOGS OF SAN FRANCISCO '= 



FROM May to September little 

 rain falls in San Francisco, but 

 every afternoon great banks of 

 fog match in from the Pacific and en- 

 wrap the houses, streets, and hills in 

 their dense folds. Ocean fogs as a rule 

 form when cool air flows over warm 

 moist surfaces ; but in the case of the 

 San Francisco sea fogs these conditions 

 are reversed, for the ocean surface tem- 

 perature is 55° Fahrenheit, while the 

 air temperature may reach 80° . Another 

 explanation, therefore, of the cause of 

 these fogs must be sought. 



A glance at the map (not reproduced) 

 shows how ocean, bay, mountain, and 

 foothills are crowded together. East of 

 San Francisco stretches a valley 450 

 miles long and 50 miles witie and level 

 as a table. In this valley the afternoon 

 temperature in summer is usually 100° 

 or over. The valley is connected by a 

 narrow water passage, the Golden Gate, 

 with the Pacific Ocean, the mean tem- 

 perature of whose waters is in this local- 

 ity about 55°. Thus within a distance 

 of 50 miles in a horizontal direction 

 there is frequentl}'- a difference of 50 

 degrees in temperature. At the same 

 time in a vertical direction there is often 

 a difference of 30 degrees in an eleva- 

 tion of half a mile. Well-marked air 

 currents, drafts, and counter-drafts are 

 therefore prevalent 



The prevailing surface air currents 

 at this season of the year are strong 

 westerly currents, but high bluffs, ridges, 

 and headlands intercept these winds at 

 such an angle that they are diverted to 

 and pour through the Golden Gate with 

 greatly increased velocity. The result 

 is that both air and water vapor are piled 

 up at this point. Mr. Mc Adie therefore 



*Aii abstract of a paper contributed to the 

 Monthly Weather Revieiv iox November, 1900, 

 by Alexander G. McAdie, forecast official of 

 the U. S. Weather Bureau at San Francisco. 



concludes that the summer afternoon 

 fogs of the San Francisco Bay region are 

 probably due to mixture, rather than to 

 radiation or expansion. They are the 

 result of sharp temperature contrasts at 

 the boundaries of air currents having 

 different temperatures, humidities, and . 

 velocities. In originating and directing 

 these air currents the peculiar contours 

 of the land also play an important part. 



The fog outside the Heads may extend 

 over an area 10 miles square and reaches 

 to a height of about half a mile. If it 

 were solidly packed its bulk would thus 

 be 50 cubic miles. As a cubic foot of 

 the fog at its average dew-point tem- 

 perature, 51° F., weighs 4.222 grains, 

 a fair estimate of its total weight, allow- 

 ing for wide swaths or channels fog 

 free, is 1,000,000 tons. This immense 

 volume is carried through the Golden 

 Gate by westerly winds blowing 22 miles 

 an hour, from i to 5 p. m. on summer 

 afternoons. 



The United States Weather Bureau 

 maintains a station on Mt. Tamalpais, 

 which is about half a mile above sea- 

 level and thus above the fog, another 

 in the city of San Francisco, where the 

 fog converges, and a third station at 

 Point Reyes, the center of origin of the 

 fog. Mt. Tamalpais is about 25 miles 

 from Point Reyes and 10 miles from 

 San Francisco. 



The differences in the temperature 

 and humidity of these three stations is 

 most marked. The highest tempera- 

 ture recorded on the mountain during 

 the year 1899 was 96°, on July 18; the 

 maximum temperature on the same day 

 at San Francisco was 66°, and at Point 

 Re3'es 52°. That is, on the mountain 

 it was 30 degrees hotter than in the 

 city and 44 degrees hotter than at Point 

 Reyes. The mean annual temperature 

 of the three stations is, however, about 

 the same for all, 55°, which is also" the 



